With recent development of portable electronic devices, electric vehicles and large capacity power storage systems, demands for large capacity batteries have arisen. A lithium-sulfur battery is a secondary battery using a sulfur series material having sulfur-sulfur bonds (S—S bonds) as a positive electrode active material and using lithium metal as a negative electrode active material, and sulfur, a main material of a positive electrode active material, has advantages of being very abundant in resources, having no toxicity and having a low atomic weight.
In addition, a lithium-sulfur battery has theoretical discharge capacity of 1672 mAh/g-sulfur and theoretical energy density of 2,600 Wh/kg, which is very high compared to theoretical energy density of other battery systems currently studied (Ni-MH battery: 450 Wh/kg, Li—FeS battery: currently studied (Ni-MH battery: 450 Wh/kg, Li—FeS battery: 480 Wh/kg, Li—MnO2 battery: 1,000 Wh/kg, Na—S battery: 800 Wh/kg), and therefore, has received attention as a battery having a high energy density property.
However, a lithium-sulfur battery has not been commercialized so far due to a low sulfur utilization rate and thereby failing to secure sufficient capacity as theoretical capacity, and a battery short circuit problem caused by dendrite formation of a lithium metal electrode. In view of the above, positive electrode materials having increased sulfur impregnation, liquid electrolytes capable of increasing a sulfur utilization rate, and the like have been developed in order to resolve such problems.
As a liquid electrolyte solvent of a lithium-sulfur battery, a mixed solvent of 1,3-dioxolane (DOL) and 1,2-dimethoxyethane (DME) has been currently used most often. A liquid electrolyte using the solvent has excellent properties in terms of a sulfur utilization rate. However, from experimental results of the inventors of the present invention, a swelling phenomenon, in which gas is generated inside and the battery swells up while operating the battery using the liquid electrolyte, was observed. Such a swelling phenomenon causes liquid electrolyte depletion, and battery deformation, and also causes active material deintercalation from an electrode resulting in a problem of declining battery performance.
Causes and generation mechanism of such a swelling phenomenon caused by gas generation inside a battery have not yet been identified, and accordingly, there are no countermeasures as well.